Optical communication systems which operate in the wavelength range between about 1100 and 1700 nanometers (nm) are of potentially great importance because the dispersion and losses in an optical fiber are typically very low in this wavelength range. Heterojunction devices incorporating binary III-V alloys and solid solutions of these alloys have been found to be particularly useful for this application because their electronic bandgaps occur in this wavelength range and lattice-matched heterojunctions can be obtained by compositional variation. In particular ternary and quaternary alloys of InGaAsP on an InP substrate have been found to be useful materials for both light emitters and detectors.
Problems which affect the performance of avalanche photodetectors using these materials include bulk tunneling currents which occur at electric fields of the order of 1.5.times.10.sup.5 V/cm in the ternary and quaternary compounds used for the light absorptive region, edge breakdown, and multiplication of surface leakage currents at the junction periphery. The tunneling has been reduced by locating the P-N junction with its high electric field in a wide bandgap material separated from the light absorptive region in the narrower bandgap material.
Edge breakdown and surface leakage currents have been reduced by the use of surface contouring of the detector sidewalls. However, the electric field reduction at the surface may be small with the result that the surface leakage current may still undergo multiplication. Therefore, it is desirable to find other device structures which will reduce the electric field at the junction periphery to an acceptable level.